1. Field of the Invention
This invention relates to a structure of a pn junction type light emitting diode using SiC, and more particularly to a blue light emitting diode with a high luminance.
2. Prior Art
Light emitting diodes, which constitute a small light source that can provide a stable and high electroluminescence by consuming only a small amount of power, can be adopted as a light source for reading information records that can be used in each kind of a display device. Visible light emitting diodes so far widely produced for practical use can emit visible light with high luminance ranging from red to green light. Compared with such light emitting diodes, blue light emitting diode is still low in luminance and are not widely produced for practical use.
In general, the color of light emitted from a light emitting diode depends on the semiconductor material therefor. Semiconductor materials to be used for light emitting diodes capable of emitting visible light with a short wavelength are limited to silicon carbide (SiC) which is a IV--IV group compound semiconductor, gallium nitride (GaN) which is a III-V group compound semiconductor, and zinc sulfide (ZnS) and zinc selenide (ZnSe) which are II-IV group compound semiconductors. With the use of these semiconductor materials, extensive research has been conducted in order to develop light emitting diodes capable of emitting visible light with a short wavelength. However, mass production of such light emitting diodes with brightness and stability sufficient for practical use has not been realized.
For the structure of light emitting diodes, a pn junction structure is most suited because electrons and holes as carriers can be injected in a light emitting region with high efficiency. However, among the above-mentioned semiconductor materials for light emitting diodes capable of emitting visible light with a short wavelength, it is impossible to use any of GaN, ZnS and ZnSe semiconductors for the production of pn junction light emitting diodes. This is because it is difficult to obtain p-type crystals from these semiconductor materials, or even if these crystals are obtained, they high resistance and are very unstable. Therefore, a metal-insulator-semiconductor (MIS) structure using a thin insulating layer or high resistive layer as an insulator has been employed instead of a pn junction structure. However, light emitting diodes with such an MIS structure have the disadvantages of having non-uniform device characteristics and of providing unstable light emission.
On the other hand, it is possible to use silicon carbide as a material for light emitting diodes of the pn junction type, because both p-type crystals and n-type crystals can readily be obtained. Many reports have already be made on blue light emitting diodes of the pn junction type using silicon carbide grown by liquid phase epitaxy (LPE). (See e.g., M. Ikeda, T, Hayakawa, S. Yamagiwa, H. Matsumani, and T. Tanaka, Journal of Applied Physics, Vol. 50, No. 12, pp. 8215-8225, 1979).
However, conventional blue light emitting diodes produced by liquid phase epitaxy, as described above, can only provide light emission with a brightness of 15 mcd or lower under the operation condition of 20 mA. The principal cause for this low brightness is considered to be as follows. The growth temperature is as high as 1700.degree. C. to 1800.degree. C., so that the crystal growth of silicon carbide takes place in active molten silicon, thereby making it difficult to accurately control the crystal growth, and also having a great possibility that unnecessary impurities will enter the growing crystals. Furthermore, there is the disadvantage that the use of liquid phase epitaxy cannot allow the mass production of blue light emitting diodes.
Under these circumstances, inventors of this invention has recently invented a method using the chemical vapor deposition (CVD) process for manufacturing in large quantity pn junction type light emitting diodes that can emit high luminance blue visible light with good controllability. (Refer to Japanese Laid-Open Patent No. 129918/1989.)
Since semiconductors using SiC is of indirect transition type, it is particularly important to reduce the disappearance ratio of injected carriers through the non-radiative recombination process to embody a high luminance blue light emitting diode using SiC. The non-radiative recombination process results from the lattice defects at the SiC pn junction portion forming a pn junction and incomplete crystallinity caused by, for example, impurities. It is important to form a good-quality SiC layer.
As shown in FIG. 15, conventional counterparts of blue light emitting diodes using SiC can be manufactured by sequentially forming a n-type SiC layer 240 and a p-type SiC layer 350 on a n-type substrate 12 with favorable light transmittance. Reference numerals 60 and 70 in FIG. 15 designate an electrode. The n-type SiC substrate needs to be low in resistance, because pn junction type diodes have a serial resistance. The substrate exhibits a resistivity of 1 .OMEGA..multidot.cm or less and electron concentration of 1.times.10.sup.17 cm.sup.-3 or more.
Many lattice defects involved in impurity-doped SiC single-crystal substrate for providing a low resistance inevitably results in exhibiting the same lattice defects in a pn junction grown layer overlaying the same substrate, which causes a non-luminescent recombination process, thereby inhibiting the embodiment of a high luminance blue light emitting diode such that it can be widely produced for practical use.